The present invention relates to cellular communication systems and more particularly to such systems in which base station transmit diversity is employed.
In cellular telephone communication systems, radio communication occurs between fixed equipment at a base station (cell sites) and mobile units within the cell. The signal path from the base to the mobile units is referred to as the "forward" path, while that in the opposite direction is the "reverse" path. Cellular systems are limited by the capability of mobile telephones to receive information in the presence of interference. The key performance measure is the minimum ratio of signal power to interference power that permits acceptable communication quality.
In one method of compliance with Telecommunications Industry Association (TIA) standards, an equalizer is required to be integrated into the mobile unit circuitry. The equalizer must be capable of operating well when receiving signals through two distinct paths separated in time by up to 41 .mu.S (a symbol duration). The intent of the equalizer is to mitigate the effects of delay spread as it arises in the transmission environment of cellular telephones. Delay spread is the period over which a signal transmitted at an instant is spread over time by the communication channel.
For appropriate equalizer architectures, excellent performance occurs when the channel exhibits particular characteristics. Specifically, the equalizer performs very well when the channel appears to have consisted of two paths of equal average power, separated in time by a symbol duration. This improved performance results from a combination of two factors: (a) the diversity between the two paths (i.e., the low probability that they will fade simultaneously relative to the individual probabilities of fading), and (b) the minimal level of Inter-Symbol Interference (ISI) that occurs when the paths are separated by exactly a symbol duration.
In typical operating environments, it is possible that paths with the desired characteristics will occur naturally. Unfortunately, such circumstances rarely arise, and delay spreads are typically negligible in comparison to the symbol duration.
In previous system base station equipment, diversity reception is typically used to improve mobile-to-base (reverse) channel performance. In a previously filed patent application Ser. No. 08/053,173 filed Apr. 26, 1993 entitled "TIME DIVISION MULTIPLE ACCESS CELLULAR COMMUNICATION SYSTEM AND METHOD EMPLOYING BASE STATION DIVERSITY TRANSMISSION" by John Watson et al., a cellular system is disclosed in which forward diversity signal transmission is employed from a base station to improve system communication quality.
In the Watson diversity transmission cellular system, forward diversity transmission is continuously employed to achieve improved system communication quality. Normally, only a relatively small percentage of total system communications are sufficiently degraded in quality that a need exists for quality enhancement by forward diversity transmission. Accordingly, the Watson system employs greater hardware and software resources than might otherwise be required if only those system communications requiring quality enhancement were to be conducted with forward diversity transmission.